In general, all unsaturated alcohols belong to the class of alcohols; they have in their structure one or more corresponding functional hydroxo groups. They are distinguished only by the presence in the molecule of multiple (double, triple) bonds. Thus unsaturated alcohols combine the properties of both unsaturated hydrocarbons and ordinary alcohols.
Structure
As a rule, the functional hydroxo group must be located at the saturated (i.e., only with single bonds) carbon atom (the carbon atom located next to the functional group of the compound is called alpha carbon). Such alcohols have all the standard properties of their ultimate neighbors. The simplest unsaturated alcohol with saturated alpha carbon is allyl alcohol or propendiol.
Enols
Alcohols with an -OH group located near unsaturated carbon are called enols. Almost all of them are unstable and upon formation almost immediately rearrange themselves into the corresponding ketones. A small part, however, remains in its original form, but it is quite small. In this case, they talk about keto-enol tautomerism: the substance contains two so-called tautomers at the same time: one has a hydrogen atom near oxygen, and this is enol, and the other hydrogen has moved to carbon, and this is ketone (carbonyl compound).
In most substances of a similar structure, the enol content is a fraction of a percent. However, there are some compounds in which, due to certain substituents at the carbon atom directly bonded to the oxygen of the hydroxo group, the relative stability of enol can be achieved. For example, in acetylacetone the percentage of enol tautomers reaches 76.
The first in the series of enols is vinyl alcohol. In keto-enol tautomerism, acetaldehyde corresponds to it.
Chemical properties
Since unsaturated alcohols contain, as it were, two functional groups at once, the set of their reactions is also a combination of the properties of two classes of compounds. In a multiple connection, they, like all unsaturated hydrocarbons, react with the addition of halogens, hydrogen, hydrogen halides and other substances that form electrophilic particles. Epoxides can also form (when oxidized with atmospheric oxygen on a silver catalyst). Also, in the double group, unsaturated alcohols can attach additional hydroxyl groups to turn into di-, trihydric alcohols. The hydroxyl group itself enters into its typical reactions: oxidation (to the corresponding carbonyl compound, and then carboxylic acid), substitution with halogen, and the formation of ethers and esters.
Being in nature
Unsaturated alcohols are found in many parts of the living world. Most often they are there in the form of esters - compounds consisting of parts of alcohol and carboxylic acid. For example, cinnamon alcohol is contained (in the form of esters of acetate and cinnamate) in hyacinth, cassia, and other odorous oils, as well as in resins in Styrax trees and in Peruvian balsam - resins from miroxilon trees. It is widely used in the perfume industry as various flavors and fragrances.
Retinol acetate - well-known vitamin A. 3-hexenol-1 - cyclic unsaturated alcohol - in the composition of essential oils of green parts of plants gives the latter a characteristic smell.
Also, for example, the well-known cholesterol is an alcohol with a very complex formula, which also contains multiple bonds (that is why in some countries they prefer to call the same substance cholesterol according to the functional group). Accordingly, many substances related to cholesterol, in particular, some fatty alcohols, have a similar structure.